RESEARCH Open Access On the Ulam-Hyers stability of a quadratic functional equation Sang-Baek Lee 1 , Won-Gil Park 2 and Jae-Hyeong Bae 3* * Correspondence: jhbae@khu.ac.kr 3 Graduate School of Education, Kyung Hee University, Yongin 446- 701, Republic of Korea Full list of author information is available at the end of the article Abstract The Ulam-Hyers stability problems of the following quadratic equation r 2 f  x + y r  + r 2 f  x − y r  =2f (x)+2f (y), where r is a nonzero rational number, shall be treated. The case r = 2 was introduced by J. M. Rassias in 1999. Furthermore, we prove the stability of the quadratic equation by using the fixed point method. 2010 Mathematics Subject Classification: 39B22; 39B52; 39B72 . Keywords: Hyers-Ulam stability, quadratic function 1. Introduction In 1940, Ulam [1] proposed the general Ulam stability problem. In 1941, this problem was solved by Hyers [2] for the case of Banach spaces. Thereafter, this type of stability is called the Ulam-Hyers stability. In 1950, Aoki [3] provided a generalization of the Ulam-Hyers stability of mappings by considering variables. For more general function case, the reader is referred to Forti [4] and Găvruta [5]. Let X be a real normed space and Y be a real Banach space in the case of functional inequalities, as well as let X and Y be real linear spaces in the case of functional equa- tions. The quadratic function f(x)=cx 2 (x Î ℝ), where c is a real constant, clearly satis- fies the functional equation f (x + y)+f (x − y)=2f (x)+2f (y). (1:1) Hence, the above equation i s called the quadratic functional equation. In particular, every solution f : X ® Y of equation (1.1) is said to be a quadratic mapping. In 1983, Skof [6] obtained the first result on the Ulam-Hyers stability of equation (1.1). In 1989, Aczel and Dhombres [7] obtained the general solution of Equation (1.1) for afunctionf from a real linear space over a commutative field F of characteristic 0 to the f ield F. In 1995, Kannappan [8] obtained the general solution of the functional equation f (λx + y)+f (x − λy)=(1+λ 2 )  f (x)+f (y)  . The solution of the above equation is connected with bilinear functions. In 1995, Forti [9] obtained the result on the stability theorem for a class of functional equations Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 © 2011 Bae et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is prop erly cited. including Equation (1.1). It is also the firs t result on the Ulam-Hyers stability of the quadratic functional equation. Recently, Shakeri, Saadati and Park [10] investigated the Ulam-Hyers stability of Equation (1.1) in non-Archimedean L -fuzzy normed spaces. In 1996, Rassias [11] investigated the stability problem for the Euler-Lagrange func- tional equation f (ax + by)+f (bx − ay)=(a 2 + b 2 )  f (x)+f (y)  , (1:2) where a, b are fixed nonzero reals with a 2 + b 2 ≠ 1. In 2009, Gordji and Khodaei [12] investigated the stability problem for the Euler-Lagrange functional equation f (ax + by)+f (ax − by)=2a 2 f (x)+2b 2 f (y), (1:3) where a, b are fixed integers with a, b, a±b≠ 0. In this paper, we will investigate the Ulam-Hyers stability of t he Euler-Lagrange functional equation as follows: r 2 f  x + y r  + r 2 f  x − y r  =2f (x)+2f (y), (1:4) where r is a nonzero rational number. Equation 1.4 is a special form with a = b = 1 r of Equation 1.2. Equation 1.4 is similar to Equation 1.3, but it is not a special form of Equation 1.3 since a ≠ b in Equation 1.3. In 2009, Ravi et al. [13] obtained the general solution and the Ulam-Hyers stability of the Euler-Lagrange additive-quadratic-cubic-quartic functional equation f (x + ay)+f (x − ay)=a 2 f (x + y)+a 2 f (x − y)+2(1− a 2 )f (x) + a 4 − a 2 12  f (2y)+f (−2y) − 4f (y) − 4f (−y)  (1:5) for a fixed integer a with a ≠ 0, ± 1. In [13], one can find the fact that Equation (1.1) implies Equation 1.5. Recently, Xu, Rassias and Xu [14] investigated t he stability pro- blem for Equation 1.5 in non-Archimedean normed spaces . Euler-Lagrange type func- tional equations in various spaces have been constantly studied by many authors. 2. Solution of the functional eq uation (1.4) Theorem 2.1 LetrbeanonzerorationalnumberandletXandYbevectorspaces.A mapping f : X ® Y satisfies the functional equation (1.4) if and only if it is quadratic. Proof Suppose that f satisfies Equation (1.4). Letting x = y = 0 in (1.4), we gain f(0) = 0. Putting y = 0 in (1.4), we get r 2 f  x r  = f (x) for all x Î X. By (1.4) and the above equation, we have f (x + y)+f (x − y)=r 2 f  x + y r  + r 2 f  x − y r  =2f (x)+2f (y) for all x, y Î X. Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 2 of 9 Conversely, suppose that f is quadratic. Then we have f (rx)=r 2 f (x) for all x Î X. Thus we obtain r 2 f  x + y r  + r 2 f  x − y r  = f (x + y)+f (x − y)=2f (x)+2f (y) for all x, y Î X. □ Remark 2.2 Let r be a nonzero real number and let X and Y be vector spaces. Let f : X ® Y be a mapping satisfying the functional equation (1.4). By the same reasoning as the proof of Theorem 2.1, it is quadratic. Remark 2.3 Let r be a nonzero real number and let X and Y be vector spaces. Let f : X ® Y be a quadratic mapping and let, for all x Î X, the mapping g x : ℝ ® Y given by g x (t):= f(tx)(t Î ℝ) be continuous. Then the mapping f satisfies the functional equa- tion (1.4). 3. Stability of the quadratic equation (1.4) For r = 1, the stability problem of Equation (1.4) has been investigated by Cholewa [15]. For r = 2, the stability problem of Equation (1.4) has been proved by Rassias [16]. From now on, let r be a nonzero rational number with |r| ≠ 2. In this section, we i nvestigate the generalized Hyers-Ulam stability of the functional equation (1.4) in the spirit of Găvruta. Let X be a normed space and Y a Banach space. For a mapping f : X ® Y, we define a mapping Df: X × X ® Y by Df(x, y):=r 2 f  x + y r  + r 2 f  x − y r  − 2f(x) − 2f (y) (3:1) for all x, y Î X. Assume that  : X × X ® [0, ∞) is a function satisfying (x, y):= ⎧ ⎨ ⎩  ∞ k=1  2 r  2k ϕ   r 2  k x,  r 2  k y  < ∞ if |r| > 2,  ∞ k=0  r 2  2k ϕ   2 r  k x,  2 r  k y  < ∞ if |r| < 2, (3:2) for all x, y Î X. Lemma 3.1 Let a mapping f : X ® Y satisfy f(0) = 0 and the inequality   Df(x, y)   ≤ ϕ(x, y) (3:3) for all x, y Î X. Then ⎧ ⎨ ⎩     2 r  2n f  r 2  n x  − f(x)    ≤ 1 4  n k=1  2 r  2k ϕ   r 2  k x,  r 2  k x  if |r| > 2,     r 2  2n f   2 r  n x  − f(x)    ≤ 1 4  n−1 k=0  r 2  2k ϕ   2 r  k x,  2 r  k x  if |r| < 2, (3:4) for all n Î N and x Î X. Proof Let |r| > 2. Now we are going to prove our assertion by induction on n. Repla- cing y by x in (3.3), we obtain     r 2 4 f  2 r x  − f(x)     ≤ 1 4 ϕ(x, x) (3:5) Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 3 of 9 for al l x Î X. Replacing x by r 2 x in (3.5) and multiplying 4 r 2 to the resulting inequal- ity, we have     4 r 2 f  r 2 x  − f(x)     ≤ 1 r 2 ϕ  r 2 x, r 2 x  (3:6) for all x Î X. Thus inequality (3.4) holds for n = 1. We assume that the assertion is true for a fixed natural number n. Replacing x by  r 2  n x in (3.6) and multiplying  2 r  2n to the resulting inequality, we have       2 r  2(n+1) f  r 2  n+1 x  −  2 r  2n f  r 2  n x       ≤ 1 4  2 r  2(n+1) ϕ  r 2  n+1 x,  r 2  n+1 x  (3:7) for all x Î X. Thus we have       2 r  2(n+1) f  r 2  n+1 x  − f(x)      ≤       2 r  2(n+1) f  r 2  n+1 x  −  2 r  2n f  r 2  n x       +       2 r  2n f  r 2  n x  − f(x)      ≤ 1 4 n+1  k=1  2 r  2k ϕ  r 2  k x,  r 2  k x  for all x Î X. Hence inequality (3.4) holds for all n Î N. The proof of the case |r| < 2 is similar to the above proof. □ In the following theorem we find that for some conditions there exists a true quadra- tic mapping near an approximately quadratic mapping. Theorem 3.2 Assume that a mapping f : X ® Y satisfies f(0) = 0 and inequality (3.3). Then there exists a unique quadratic mapping Q : X ® Y satisfying   f (x) − Q(x)   ≤ 1 4 (x, x) (3:8) for all x Î X. Proof Let |r| > 2. For each n Î N, define a mapping Q n : X ® Y by Q n (x):=( 2 r ) 2n f (( r 2 ) n x) for all x Î X. For each x Î X, in order to prove the conver- gence of the sequence {Q n (x)},we have to show that {Q n (x)} is a Cauchy sequence in Y. By inequality (3.7), for all integers l, m with 0 ≤ l<m, we get       2 r  2l f  r 2  l x  −  2 r  2m f  r 2  m x       ≤ 1 4 m−1  n=l  2 r  2(n+1) ϕ  r 2  n+1 x,  r 2  n+1 x  Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 4 of 9 for all x Î X.Takingl, m ® ∞ in the above in the above inequality, by inequality (3.2), we may conclude that the sequence {Q n (x)} is a Cauchy sequence in the Banach space Y for each x Î X . This implies that the sequence {Q n (x)} converges fo r each x Î X. Hence one can define a function Q : X ® Y by Q(x) := lim n→∞  2 r  2n f  r 2  n x  for all x Î X. By letting n ® ∞ in (3.4), we arrive at the formula (3.8). Now we show that Q satisfies the functional equation (1.4) for all x, y Î X. By the definition of Q,     r 2 Q  x + y r  + r 2 Q  x − y r  − 2Q(x) − 2Q(y)     = lim n→∞  2 r  2n     r 2 f  2 r  n x + y r  + r 2 f  2 r  n x − y r  −2f  r 2  n x  − 2f  r 2  n y     ≤ lim n→∞  2 r  2n ϕ  r 2  n x,  r 2  n y  =0 for all x, y Î X. Hence Q is quadratic by Theorem 2.1. It only remains to claim that Q is unique. L et Q’: X ® Y be another quadratic mapping satisfying inequality (3.8). Since Q and Q’ are quadratic mapping, we can easily show t hat Q  r 2  n x  =  r 2  2n Q ( x ) and Q   r 2  n x  =  r 2  2n Q  ( x ) for all n Î ℓ and all x Î X. Thus we see that   Q(x) − Q  (x)   ≤  2 r  2n    Q  r 2  n x  − f  r 2  n x       +  2 r  2n      f  r 2  n x  − Q   r 2  n x     ≤ 1 2  2 r  2n   r 2  n x,  r 2  n x  for all n Î N and all x Î X. By letting n ® ∞, we get that Q(x)=Q’(x) for all x Î X. The proof of the case |r| < 2 is similar to the above proof. □ Corollary 3.3 Let |r|>2and let ε, p, q Î N with p, q<2 and ε ≥ 0. If a mapping f : X ® Y satisfies f(0) = 0 and the inequality   Df(x, y)   ≤ ε(  x  p +   y   q ) for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ ε  x  p 2 p r 2−p − 4 +  x  q 2 q r 2−q − 4  for all x Î X. Corollary 3.4 Let |r|>2and let ε, s, t Î ℝ wit h s + t<2 and h ≥ 0. If a mapping f : X ® Y satisfies f(0) = 0 and the inequality   Df(x, y)   ≤ η  x  s   y   t Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 5 of 9 for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ η  x  s+t 2 s+t r 2−s−t − 4 for all x Î X. Let |r| > 2 and let ε be a nonnegative real numbe r. If a mapping f : X ® Y satisfies f (0) = 0 and the inequality   Df(x, y)   ≤ η for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ η r 2 − 4 for all x Î X. Corollary 3.5 Let |r|<2and let ε, p, q Î ℝ with p, q>2 and ε ≥ 0. If a mapping f : X ® Y satisfies f(0) = 0 and the inequality   Df(x, y)   ≤ ε(  x  p +   y   q ) for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ ε  x  p 4 − 2 p r 2−p +  x  q 4 − 2 q r 2−q  for all x Î X. Corollary 3.6 Let |r|<2and let ε, s, t Î ℝ wit h s + t>2 and h ≥ 0. If a mapping f : X ® Y satisfies f(0) = 0 and the inequality   Df(x, y)   ≤ η  x  s   y   t for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ η  x  s+t 4 − 2 s+t r 2−s−t for all x Î X. Let |r| < 2 and let h be a nonnegative real number. If a mapping f : X ® Y satisfies f (0) = 0 and the inequality   Df (x, y)   ≤ η for all x, y Î X, then there exists a unique quadratic mapping Q : X ® Y such that   f (x) − Q(x)   ≤ η 4 − r 2 for all x Î X. 4. Stability using alternative fixed point In this section, we will investigate the stability of the given quadratic functional equa- tion (3.1) using alternative fixed point. Before proceeding the proof, we will state the theorem, alternative fixed point. Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 6 of 9 Theorem 4.1 (The alternative fixed point [17,18] ) Supposethatwearegivenacom- plete generalized metric sp ace (Ω, d) and a strictly contractive mapping T : Ω ® Ω with Lipschitz constant L. Then (for each given x Î Ω), either d(T n x, T n+1 x)=∞ for all n ≥ 0 or there exists a natural number n 0 such that (1) d(T n x, T n+1 x)<∞ for all n ≥ n 0 ; (2) the sequence (T n x) is convergent to a fixed point y* of T; (3) y* is the unique fixed point of T in the set  = {y ∈ |d(T n 0 x, y) < ∞} ; (4) d(y, y ∗ ) ≤ 1 1−L d(y, Ty) for all y Î Δ. From now on, let  : X × X ® [0, ∞) be a function lim n→∞ ϕ(λ n i x, λ n i y) λ 2n i =0 (i =0,1) for all x, y Î X, where λ i = r 2 if i = 0 and λ i = 2 r if i =1. Theorem 4.2 Suppose that a mapping f : X ® Y satisfies the functional inequality   Df(x, y)   ≤ ϕ(x, y) (4:1) for all x, y Î X and f(0) = 0. If there exists L = L(i) <1 such that the function x → (x):=ϕ(x, x ) (4:2) has the property (x) ≤ L · λ 2 i ·  x λ i  (4:3) for all x Î X, then there exists a unique quadratic mapping Q : X ® Y such that the inequality   f (x) − Q(x)   ≤ L 1−i 4(1 − L) (x) (4:4) holds for all x Î X. Proof Consider the set Ω:= {g|g: X ® Y, g(0) = 0} and introduce the generalized metric d on Ω given by d(g, h)=d (g, h):=inf{k ∈ (0, ∞)|   g(x) − h(x)   ≤ k(x)forallx ∈ X} for all g, h Î Ω. It is easy to show that (Ω, d) is complete. Now we define a mapping T : Ω ® Ω by Tg(x)= 1 λ 2 i g(λ i x) for all x Î X. Note that for all g, h Î Ω, d(g, h) < k ⇒   g(x) − h(x)   ≤ k (x)forallx ∈ X ⇒     1 λ 2 i g(λ i x) − 1 λ 2 i h(λ i x)     ≤ 1 λ 2 i k (λ i x)forallx ∈ X ⇒     1 λ 2 i g(λ i x) − 1 λ 2 i h(λ i x)     ≤ Lk (x)forallx ∈ X ⇒ d(Tg, Th ) ≤ Lk. Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 7 of 9 Hence we have that d(Tg, Th) ≤ Ld(g, h) for all g, h Î Ω,thatis,T is a strictly con- tractive mapping of Ω with Lipschitz constant L. We have inequality (3.6) as in the proof of Lemma 3.1. By inequalities (3.6) and (4.3) with the case i = 0, we get       2 r  2 f  r 2 x  − f(x)      ≤ 1 r 2  r 2 x  ≤ 1 4 L ( x ) for all x, that is, d(f , Tf) ≤ L 4 = L 1 4 < ∞. Similarly, we get d(f , Tf) ≤ 1 4 = L 0 4 < ∞ for the case i = 1. In both cases we can apply the fixed point alternative and since lim n®∞ d(T n f, Q) = 0, there exists a fixed point Q of T in Ω such that Q(x) = lim n→∞ f (λ n i x) λ 2n i for all x Î X. Letting x = λ n i x, y = λ n i y in Equation (4.1) and dividing by λ 2n i ,   DQ(x, y)   = lim n→∞ Df (λ n i x, λ n i y) λ 2n i ≤ lim n→∞ ϕ(λ n i x, λ n i y) λ 2n i =0 for all x, y Î X.Thatis,Q satisfies Equation (1.4). By Theorem 2.1, Q is quadratic. Also, the fixed point alternative guarantees that such Q is the unique mapping such that ||f(x)-Q(x)|| ≤ k (x)forallx Î X and some k>0. Again using the fixed point alternative, we have d(f , Q) ≤ 1 1−L d(f , Tf) . Hence we may conclude that d(f , Q) ≤ L 1−i 4(1 − L) , which implies inequality (4.4). □ Corollary 4.3 Let p, q, s, t be real numbers such that p, q, s + t<2 or p, q, s + t>2 and let ε, h be nonnegative real numbers. Suppose that a mapping f : X ® Ysatisfies the functional inequality   Df (x, y)   ≤ ε(  x  p +   y   q )+η  x  s   y   t for all x, y Î Xandf(0) = 0. Then there exists a unique quadratic mapping Q : X ® Y such that the inequality   f (x) − Q(x)   ≤ L 1−i ε 4(1 − L)  ε(  x  p +  x  q )+η  x  s+t  holds for all x Î X, where L := max{λ p i , λ q i , λ s+t−2 i } (i =0,1) , λ 0 = r 2 if p, q, s + t <2; λ 1 = 2 r if p, q, s + t >2. Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 8 of 9 Author details 1 Department of Mathematics, Chungnam National University, Daejeon 305-764, Republic of Korea 2 Department of Mathematics Education, College of Education, Mokwon University, Daejeon 302-729, Republic of Korea 3 Graduate School of Education, Kyung Hee University, Yongin 446-701, Republic of Korea Authors’ contributions All authors contributed equally to this work. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 16 February 2011 Accepted: 6 October 2011 Published: 6 October 2011 References 1. Ulam, SM: A Collection of Mathematical Problems. Interscience Publishers, New York. 63 (1968) 2. Hyers, DH: On the stability of the linear functional equation. Proc Natl Acad Sci USA. 27, 222–224 (1941). doi:10.1073/ pnas.27.4.222 3. Aoki, T: On the stability of the linear transformation in Banach space. J Math Soc Jpn. 2,64–66 (1950). doi:10.2969/jmsj/ 00210064 4. Forti, GL: An existence and stability theorem for a class of functional equations. Stochastica. 4,23–30 (1980). doi:10.1080/17442508008833155 5. Găvruta, P: A generalization of the Hyers-Ulam-Rassias stability of approximately additive mappings. J Math Anal Appl. 184, 431–436 (1994). doi:10.1006/jmaa.1994.1211 6. Skof, F: Proprietà locali e approssimazione di operatori. 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Dacia, Cluj-Napoca (1979) (in Romanian) doi:10.1186/1029-242X-2011-79 Cite this article as: Lee et al.: On the Ulam-Hyers stability of a quadratic functional equation. Journal of Inequalities and Applications 2011 2011:79. Submit your manuscript to a journal and benefi t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Lee et al. Journal of Inequalities and Applications 2011, 2011:79 http://www.journalofinequalitiesandapplications.com/content/2011/1/79 Page 9 of 9 . also the firs t result on the Ulam-Hyers stability of the quadratic functional equation. Recently, Shakeri, Saadati and Park [10] investigated the Ulam-Hyers stability of Equation (1.1) in non-Archimedean L -fuzzy. RESEARCH Open Access On the Ulam-Hyers stability of a quadratic functional equation Sang-Baek Lee 1 , Won-Gil Park 2 and Jae-Hyeong Bae 3* * Correspondence: jhbae@khu.ac.kr 3 Graduate School of. generalized Hyers-Ulam-Rassias stability of quadratic functional equations. Abst Appl Anal 2009 (2009). Article ID 923476 13. Ravi, K, Rassias, JM, Arunkumar, M, Kodandan, R: Stability of a generalized